Physical activity and exercise have become an accepted and valued component of Cystic Fibrosis care. Regular physical activity and exercise can slow the rate of decline of pulmonary function, improve physical fitness, and enhance quality of life. However, motivating people to be more active is challenging. Supervised exercise programs are expensive and labor intensive, and adherence falls off significantly once supervision ends. Unsupervised or partially supervised programs are less costly and more flexible, but compliance can be more problematic. The primary objective of this study is to evaluate the effects of a 12-months partially supervised exercise intervention along with regular motivation on forced expiratory volume in 1 second (FEV1) in a large international group of cystic fibrosis patients. Secondary endpoints include patient reported quality of life, as well as levels of anxiety and depression, and control of blood sugar. A total of 292 patients with cystic fibrosis 12 years and older with a FEV1 ≥35% predicted will be recruited. Following baseline assessments (2 visits) patients will be randomized into an intervention and a control group. Thereafter, they will be seen every 3 months for assessments in their centre for one year (4 follow-up visits). Along with individual counseling to increase vigorous physical activity by at least 3 hours per week on each clinic visit, the intervention group will document daily exercise and inactivity time and will receive a step counter and they will record their progress with a web-based program. They will also receive monthly phone calls from the study staff. After 6 months, they will continue with the step counter and web-based program for a further 6 months. The control group will receive access to this intervention after 12 months of standardized care. Should this relatively simple program prove successful, this will be made available on a wider scale internationally.